1. Field of the Invention
The present invention relates to an aeration-less water treatment system which performs an aerobic treatment by taking in air by making use of natural downflow of sewage.
2. Description of the Related Art
Conventionally, a water treatment system which cleans sewage has been proposed.
This water treatment system, as shown in FIG. 1, comprises an anaerobic treatment tank 101 and an aerobic treatment tank 102. In the anaerobic treatment tank 101, an anaerobic lower cavity section 103, an anaerobic filter bed section 104 and an anaerobic upper overflow section 105 are arranged in the named order from the lower side to the upper side. With driving of a pump 107, sewage 106 flows through a water pipe 108 and is supplied to the anaerobic lower cavity section 103. Then, the sewage 106 flows, as an upward stream, into the anaerobic filter bed section 104.
A great number of plastic carriers are disposed within the anaerobic filter bed section 104. Anaerobic microorganisms are adhered to the surface of each plastic carrier. Organic matter in the sewage, which has entered the anaerobic filter bed section 104, comes in contact with the anaerobic microorganisms. As a result, a predetermined amount of the organic matter is decomposed and removed by the decomposing function of the anaerobic microorganisms.
When the water, from which the predetermined amount of organic matter has been decomposed and removed by the decomposing function of the anaerobic microorganisms, is filled up to the anaerobic upper overflow section 105, the water is output as anaerobic treatment tank outlet water 109, and is supplied to the upper part of the aerobic treatment tank 102 through a water pipe 110. Reference numeral 109a denotes the water surface of the anaerobic treatment tank outlet water 109.
In the aerobic treatment tank 102, an aerobic filter bed section 111 and an aerobic lower cavity section 112 are arranged in the named order from the upper side to the lower side. A diffusion pipe 113 is disposed in the aerobic lower cavity section 112. With driving of a blower 114, air is taken in through air pipes 115 and bubbles are fed into the aerobic lower cavity section 112 from the diffusion pipe 113. Thereby, aeration treatment is performed.
The water, which has been supplied from the anaerobic treatment tank 101 into the aerobic treatment tank 102, is delivered to the aerobic filter bed section 111 as a downward stream. A great number of plastic carriers are disposed in the aerobic filter bed section 111, as in the anaerobic filter bed section 104. Aerobic microorganisms are adhered to the surface of each plastic carrier. At this time, the organic matter remaining in the anaerobic treatment tank outlet water 109 flows, as a downward stream, in the aerobic filter bed section 111, and comes in contact with the aerobic microorganisms and with the air that is supplied from the diffusion pipe 113. As a result, the residual organic matter is decomposed and removed by the decomposing function of the activated aerobic microorganisms. Reference numeral 116 designates a water pipe for discharging treated water, which has been drained from the aerobic filter bed section 111, to, e.g. a subsequent disinfection process (see Jpn. Pat. Appln. KOKAI Publication No. 11-285696).
As regards the above-described water treatment system, however, the following problems have been pointed out.    (1) The running cost of the blower 114 is high.
The diffusion pipe 113 is disposed in the aerobic lower cavity section 112, and bubbles are fed into the aerobic lower cavity section 112 from the diffusion pipe 113 by driving the blower 114. Since the driving energy for the blower 114 is large and the blower 114 needs to be driven at all times, the running cost of the blower 114 is high. In other words, it is difficult to perform water treatment at low running cost without driving the blower 114. Normally, the ratio of the driving energy cost of the blower 114 to the total running cost of the sewage treatment is about 50% to 70%. There is a demand for a decrease in the running cost of the blower 114.    (2) The treatment performance of the aerobic treatment becomes unstable.
In the water treatment system, if the blower 114 is dispensed with or if the air amount of the blower 114 is reduced, the oxygen in the aerobic lower cavity section 112 becomes deficient. As a result, proliferation of aerobic microorganisms, which require oxygen for their survival, would be suppressed, the microorganisms would be killed, and the treatment performance of the aerobic treatment tank 102 would considerably deteriorate.    (3) The process of reducing nutrient salts, such as nitrogen and phosphorus, is difficult to carry out only with the aerobic treatment.
When nitrogen, phosphorus, etc. are to be treated, it is necessary to adopt a method using microorganic reactions based on a nitrification reaction by anaerobic treatment and a denitrification reaction by aerobic treatment, or a method using a photosynthesis reaction. In this case, a circulation process from aerobic treatment to anaerobic treatment is needed. In the above-described water treatment process, however, if the circulation process from aerobic treatment to anaerobic treatment is adopted, aerobic microorganisms would mix in the anaerobic treatment tank 101, leading to degradation in anaerobic treatment. Thus, the circulation process from aerobic treatment to anaerobic treatment cannot be adopted, and as a result the process of reducing nitrogen, phosphorus, etc. is difficult to perform.
In the case of adopting the method using the photosynthesis reaction, the upper part of the treatment tank may be opened. However, only the opened part is irradiated with light, and no photosynthesis reaction takes place in other parts excluding the opened part. As a result, it is difficult to treat nutrient salts such as nitrogen and phosphorus.